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During animal development, a single fertilized egg forms a complete organism with tens to trillions of cells that encompass a large variety of cell types. Cell cycle regulation is therefore at the center of development and needs to be carried out in close coordination with cell differentiation, migration, and death, as well as tissue formation, morphogenesis, and homeostasis. The timing and frequency of cell divisions are controlled by complex combinations of external and cell-intrinsic signals that vary throughout development. Insight into how such controls determine in vivo cell division patterns has come from studies in various genetic model systems. The nematode Caenorhabditis elegans has only about 1000 somatic cells and approximately twice as many germ cells in the adult hermaphrodite. Despite the relatively small number of cells, C. elegans has diverse tissues, including intestine, nerves, striated and smooth muscle, and skin. C. elegans is unique as a model organism for studies of the cell cycle because the somatic cell lineage is invariant. Somatic cells divide at set times during development to produce daughter cells that adopt reproducible developmental fates. Studies in C. elegans have allowed the identification of conserved cell cycle regulators and provided insights into how cell cycle regulation varies between tissues. In this review, we focus on the regulation of the cell cycle in the context of C. elegans development, with reference to other systems, with the goal of better understanding how cell cycle regulation is linked to animal development in general., (Copyright © 2019 by the Genetics Society of America.)

Cell proliferation and differentiation show a remarkable inverse relationship. Precursor cells continue division before acquiring a fully differentiated state, while terminal differentiation usually coincides with proliferation arrest and permanent exit from the division cycle. Mechanistic insight in the temporal coordination between cell cycle exit and differentiation has come from studies of cells in culture and genetic animal models. As initially described for skeletal muscle differentiation, temporal coordination involves mutual antagonism between cyclin-dependent kinases that promote cell cycle entry and transcription factors that induce tissue-specific gene expression. Recent insights highlight the contribution of chromatin-regulating complexes that act in conjunction with the transcription factors and determine their activity. In particular SWI/SNF chromatin remodelers contribute to dual regulation of cell cycle and tissue-specific gene expression during terminal differentiation. We review the concerted regulation of the cell cycle and cell type-specific transcription, and discuss common mutations in human cancer that emphasize the clinical importance of proliferation versus differentiation control.

Cyclin-dependent kinases 4 and 6 (CDK4/6) in complex with D-type cyclins promote cell cycle entry. Most human cancers contain overactive CDK4/6-cyclin D, and CDK4/6-specific inhibitors are promising anti-cancer therapeutics. Here, we investigate the critical functions of CDK4/6-cyclin D kinases, starting from an unbiased screen in the nematode Caenorhabditis elegans. We found that simultaneous mutation of lin-35, a retinoblastoma (Rb)-related gene, and fzr-1, an orthologue to the APC/C co-activator Cdh1, completely eliminates the essential requirement of CDK4/6-cyclin D (CDK-4/CYD-1) in C. elegans. CDK-4/CYD-1 phosphorylates specific residues in the LIN-35 Rb spacer domain and FZR-1 amino terminus, resembling inactivating phosphorylations of the human proteins. In human breast cancer cells, simultaneous knockdown of Rb and FZR1 synergistically bypasses cell division arrest induced by the CDK4/6-specific inhibitor PD-0332991. Our data identify FZR1 as a candidate CDK4/6-cyclin D substrate and point to an APC/C(FZR1) activity as an important determinant in response to CDK4/6-inhibitors.

Caenorhabditis elegans is an important system for the study of cell cycle regulation in the context of animal development. One of the most powerful features of C. elegans is the invariant cell lineage in which somatic cells initiate cell division at specific times within the developmental program. The cell lineage is fully known and provides the foundation for the analysis of cell cycle progression at single-cell resolution in a multicellular organism. In this chapter, we describe the different types of cell cycles observed in C. elegans, and provide methods and reagents for the analysis of cell cycle progression as well as specific cell cycle phases. We also provide strategies for the analysis and proper interpretation of cell cycle and checkpoint mutants., (Copyright © 2012 Elsevier Inc. All rights reserved.)

Cell proliferation and differentiation are regulated in a highly coordinated and inverse manner during development and tissue homeostasis. Terminal differentiation usually coincides with cell cycle exit and is thought to engage stable transcriptional repression of cell cycle genes. Here, we examine the robustness of the post-mitotic state, using Caenorhabditis elegans muscle cells as a model. We found that expression of a G1 Cyclin and CDK initiates cell cycle re-entry in muscle cells without interfering with the differentiated state. Cyclin D/CDK4 (CYD-1/CDK-4) expression was sufficient to induce DNA synthesis in muscle cells, in contrast to Cyclin E/CDK2 (CYE-1/CDK-2), which triggered mitotic events. Tissue-specific gene-expression profiling and single molecule FISH experiments revealed that Cyclin D and E kinases activate an extensive and overlapping set of cell cycle genes in muscle, yet failed to induce some key activators of G1/S progression. Surprisingly, CYD-1/CDK-4 also induced an additional set of genes primarily associated with growth and metabolism, which were not activated by CYE-1/CDK-2. Moreover, CYD-1/CDK-4 expression also down-regulated a large number of genes enriched for catabolic functions. These results highlight distinct functions for the two G1 Cyclin/CDK complexes and reveal a previously unknown activity of Cyclin D/CDK-4 in regulating metabolic gene expression. Furthermore, our data demonstrate that many cell cycle genes can still be transcriptionally induced in post-mitotic muscle cells, while maintenance of the post-mitotic state might depend on stable repression of a limited number of critical cell cycle regulators., Competing Interests: The authors have declared that no competing interests exist.

DNA replication and its connection to M phase restraint are studied extensively at the level of single cells but rarely in the context of a developing animal. C. elegans lin-6 mutants lack DNA synthesis in postembryonic somatic cell lineages, while entry into mitosis continues. These mutants grow slowly and either die during larval development or develop into sterile adults. We found that lin-6 corresponds to mcm-4 and encodes an evolutionarily conserved component of the MCM2-7 pre-RC and replicative helicase complex. The MCM-4 protein is expressed in all dividing cells during embryonic and postembryonic development and associates with chromatin in late anaphase. Induction of cell cycle entry and differentiation continues in developing mcm-4 larvae, even in cells that went through abortive division. In contrast to somatic cells in mcm-4 mutants, the gonad continues DNA replication and cell division until late larval development. Expression of MCM-4 in the epidermis (also known as hypodermis) is sufficient to rescue the growth retardation and lethality of mcm-4 mutants. While the somatic gonad and germline show substantial ability to cope with lack of zygotic mcm-4 function, mcm-4 is specifically required in the epidermis for growth and survival of the whole organism. Thus, C. elegans mcm-4 has conserved functions in DNA replication and replication checkpoint control but also shows unexpected tissue-specific requirements., (Copyright © 2010 Elsevier Inc. All rights reserved.)

How cells acquire specific fates in conjunction with cell division is a major developmental question. In a recent issue of Nature, Caro and colleagues describe the Arabidopsis protein GEM, which interacts both with DNA-replication and transcriptional regulators (Caro et al., 2007). The results are surprisingly reminiscent of the dual functions reported for Geminin in animals.

Cell-division control affects many aspects of development. Caenorhabditis elegans cell-cycle genes have been identified over the past decade, including at least two distinct Cyclin-Dependent Kinases (CDKs), their cyclin partners, positive and negative regulators, and downstream targets. The balance between CDK activation and inactivation determines whether cells proceed through G1 into S phase, and from G2 to M, through regulatory mechanisms that are conserved in more complex eukaryotes. The challenge is to expand our understanding of the basic cell cycle into a comprehensive regulatory network that incorporates environmental factors and coordinates cell division with growth, differentiation and tissue formation during development. Results from several studies indicate a critical role for CKI-1, a CDK inhibitor of the Cip/Kip family, in the temporal control of cell division, potentially acting downstream of heterochronic genes and dauer regulatory pathways.

The nematode C. elegans provides an animal model that allows for genetic dissection of cell cycle regulation in the context of development. Processes such as progression through meiotic prophase, spindle function, chromosome segregation, and cytokinesis are attractive topics to study in the worm, because of their relative visibility and susceptibility to RNA interference. In addition, the reproducible cell lineage greatly helps the identification and characterization of mutants that affect cell cycle entry or exit during larval development. This chapter looks at cell division as an integral part of C. elegans development, describes how it can be studied in the worm and summarizes some of the conserved cell cycle components described to date.

The key cell-cycle regulator Cdc2 belongs to a family of cyclin-dependent kinases in higher eukaryotes. Dominant-negative mutations were used to address the requirement for kinases of this family in progression through the human cell cycle. A dominant-negative Cdc2 mutant arrested cells at the G2 to M phase transition, whereas mutants of the cyclin-dependent kinases Cdk2 and Cdk3 caused a G1 block. The mutant phenotypes were specifically rescued by the corresponding wild-type kinases. These data reveal that Cdk3, in addition to Cdc2 and Cdk2, executes a distinct and essential function in the mammalian cell cycle.